Due to recent technological advances in their production processes and their inherent low-cost and energy-efficient nature, LEDs are rapidly becoming one of the most, if not the most, popular light sources for illumination purposes. Additionally, because LEDs are semiconductor devices, they are much more resilient and damage resistant than the currently popular incandescent light bulbs and fluorescent tubes.
In view of these beneficial attributes of LEDs, many manufacturers of lighting equipment are focusing their efforts on providing lighting devices, e.g., luminaires, etc., that incorporate LED light sources instead of incandescent or compact fluorescent light sources.
An LED is a semiconductor device that produces light when electric current is passed through it in a particular direction. The light is produced due to a conversion of energy that occurs within the specific semiconductor material used to fabricate the particular LED. Research conducted over the last few decades has resulted in the ability to produce LEDs that emit a wide spectrum of monochromatic colors including basic Red, Blue and Green, as well as a myriad of colors in-between these basic three colors.
There are many significant benefits to working with LEDs; energy conservation is one of the most widely known. A direct comparison of LEDs to the other prevailing lighting technologies, such as incandescent and fluorescent, indicates the energy savings that can be realized. Specifically, incandescent light sources use the most energy, fluorescent light sources use the second most, and LEDs use the least amount of energy and, thus, are the most energy-efficient of the three.
As mentioned, unlike incandescent and fluorescent light sources, LEDs can emit light of many monochromatic colors and are not limited to white light or the various shades of white. That is, depending on the particular semiconductor material used, as well as several other factors, including the manufacturing process used, the particular packaging, and others, LEDs typically emit light in a narrow band of colors, wavelengths. For example, Table 1 below provides the band of wavelengths associated with the seven recognized colors of the rainbow.
TABLE 1The Visible Light SpectrumColorWavelength (nm)Red625-740Orange590-625Yellow565-590Green520-565Cyan500-520Blue435-500Violet380-435
“White LEDs” are often made with a blue “pump” LED and phosphors to down-convert the blue spectrum to visible white light. Based on present fabrication techniques, it is difficult to produce LEDs that emit a consistent band of wavelengths, i.e., at a specific color temperature, or that maintain the same light emission characteristics over time. This is one of the reasons it is often required to mix the light from two or more separate LEDs, such that the resulting band of wavelengths will have a dominant wavelength, or peak, at or near the desired wavelength at a desired correlated color temperature (CCT).
With respect to white light and/or the various shades of white, which are commonly used for illumination, the emitted band, or spectrum, of wavelengths emitted by an LED source is fairly broad. This is due to the inclusion of phosphor with different purities, particle sizes, and layer thicknesses that mix to produce white light.
Another known method for generating white light using LED light sources is to mix the respective light from three or more separate LED sources, for example Red, Green and Blue (RGB), or Red, Green, Blue and White (RGBW). To accommodate for the differences in the emitted light wavelength(s), LEDs are “binned” and packaged to balance the variations of the material and the manufacturing process with the needs of the lighting industry. Lighting-class LEDs are driven by application requirements and industry standards, including color consistency and color and lumen maintenance. Similar to the manner in which traditional incandescent and gas tube lamps are sold by brightness (e.g., as indicated by wattage) and color (warm or cool white), LEDs are binned for brightness (luminous flux) and color parameters (chromaticity).
Several performance requirements and standards for LED lighting applications have been established in the U.S. and elsewhere. The first of these standards was a 2007 industrial policy that mandated illumination technology for LED lamp requirements. This was the “ENERGY STAR® “Program Requirements for Solid State Lighting Luminaires.” Several additional policies/standards were released subsequently and each of these documents includes requirements for CCT, color rendering index (CRI), lumen and color maintenance for an ENERGY-STAR-approved LED illumination products.
The “temperature” in the CCT measurement refers to black-body radiation, i.e., the light emitted by a solid object, such as metal, heated to the point of incandescence. The unit for CCT measurement is expressed in degrees K (Kelvin), a standard measurement of absolute temperature. Specifically, as a black-body gets hotter, the light it emits progresses through a sequence of colors, e.g., from red to orange to yellow to white to blue. The sequence of colors defines a curve within a color space. FIG. 1 shows the CIE 1931 color space, created by the International Commission on Illumination (CIE) to define the entire range of colors visible to the average viewer, with the black-body curve, also referred to as the Planckian locus, superimposed on it.
An incandescent lamp emits light with a color of roughly 2700 K which, as shown in FIG. 1, is toward the orange or reddish end of the scale. Because an incandescent bulb operates by heating a filament, which emits light when it reaches a certain temperature, the temperature of the filament is also the color temperature of the light.
Due to specialized testing equipment that measures the spectral component(s) of light, it is possible to define color temperatures for non-incandescent white light sources, such as fluorescent tubes and LEDs. However, because LEDs are semiconductor devices and do not operate by heating a metal filament, the actual temperature of an LED source is much lower than an incandescent bulb emitting the same color temperature of light. For example, an LED emitting light measured to be 2700 K may actually only heat up to around 80° C.
The American National Standards Institute (ANSI) issued a standard for the color of light waves emitted from LED light sources. Specifically, chromaticity standard C78.377A, published in 2008, defines eight nominal CCTs that range from 2700 K (referred to as “warm” light) to 6500 K (“daylight”).
Referring to FIG. 2, each of the eight nominal CCTs is represented by a quadrilateral the interior of which defines the allowable variations both along and perpendicular to the Planckian locus, or black-body curve. The respective allowable variations corresponding to each nominal CCT are specified in the ANSI standard (i.e., ANSI C78.377A).
As can be seen by viewing the quadrilaterals in FIG. 2 and referring back to FIG. 1 as well, variations that lie along the black-body curve make a light source appear more reddish as the X chromaticity value increases, when the change is toward the right of the black-body curve, or more bluish as X decreases, i.e., when the change is toward the left of the black-body curve. Variations above and below the black-body curve make a light source appear more greenish as the Y value increases, or pinkish as the Y value decreases.
Variations along the black-body curve are measured in degrees K, while variations perpendicular to the black-body curve are notated as Duv. Duv ranges are defined on the CIE 1976 color space, rather than the 1931 color space, because the 1976 color space (also known as the CIELUV color space) is better suited for evaluating color differences of light sources because it uses a uniform scale in which a distance measured anywhere on the color space represents the same degree of difference in color.
The axes of the CIE 1976 color space are u′ and v′, instead of x and y. Duv measures the distance from the black-body curve, and therefore the degree of color change. Positive Duv values are above the curve, while negative Duv values are below the curve.
Table 2 below provides the allowable variations in CCT value, i.e., along the black-body curve, and the allowable variations in Duv value, i.e., perpendicular to the black-body curve, for each of the eight nominal CCT values, according to ANSI C78.377A). For example, as shown, the 4500 K quadrilateral covers CCT values, or temperatures, ranging from 4260 K to 4746 K along the black-body curve. Additionally, the 4500 K quadrilateral extends up to 0.007 above the curve to 0.005 below the curve.
TABLE 2Nominal CCT (ANSI C78.377A)CCTCCT RangeDuv Range2700 K2725 K ± 1450.000 ± 0.0063000 K3045 K ± 1750.000 ± 0.0063500 K3465 K ± 2450.000 ± 0.0064000 K3985 K ± 2750.001 ± 0.0064500 K4503 K ± 2430.001 ± 0.0065000 K5028 K ± 2830.002 ± 0.0065700 K5665 K ± 3550.002 ± 0.0066500 K6530 K ± 5100.003 ± 0.006
In view of the specific issues related to LED lighting and, in particular, illumination using LED sources, it is desired to provide a method and an associated device that provides for accurate and cost-effective grouping of separate LED sources in order to achieve a specific desired light output effect. For example, as explained in more detail below, because currently available individual LED devices do not all emit consistent white light that is ideal for illumination applications and, thus, two or more LED sources are typically mixed to achieve the desired color of light emission, a method of accurately selecting individual LEDs that will combine to most closely achieve that color is desired.
Further, because there are relatively wide variances in color CCT value) and sometimes noticeable variations in luminous flux from one LED to the next, even between LEDs produced from the same semiconductor wafer, LED manufacturers charge a premium for offering groupings of LEDs that have been tested and grouped into “bins” such that all of the LEDs within a given bin are known to have a color value within a particular range and a flux within a particular range. The smaller the range in these parameters leads to a correspondingly higher premium charged by the LED manufacturer. Accordingly, it is further desired to provide a method and device that enables a lighting device designer or manufacturer that purchases LEDs from LED manufacturers to avoid or minimize the premiums charged for tightly specified LEDs.